The present invention is directed to methods and apparatus for the design and fabrication of graded refractive index (rugate) optical filters as well as to complex rugate filters having prespecified refractive index versus thickness profiles.
The term "filter" is used herein in a comprehensive sense to include active and passive absorption devices, transmission devices and/or reflection devices. Typically, such optical filter devices function in a wavelength range which is at least in part within the visible, near UV and/or near IR electromagnetic spectrum range.
Optical filters are conventionally provided by depositing alternating layers of dielectric materials of different refractive index, having the respective layers being of a thickness determined in the desired constructive or destructive interference desired at the wavelength(s) of interest. In order to provide various optical filter functions, a plurality of dielectric stacks each of different optical filtering properties are conventionally provided in adjacent array. However, such composite optical filter devices have disadvantages of undesirably low efficiency and discrimination capability. Improved optical filters of increased efficiency, filter function complexity and/or discrimination capacity would be desirable.
Optical filters are conventionally manufactured by depositing dielectric layers on a suitable substrate, such as a reflective or transparent substrate. The monitoring and controlling of the deposition process is a limiting factor in the manufacture and performance of optical filters.
Monitoring of deposit thickness and refractive index during deposition have utilized reflectance monitoring combined with a crystal thickness monitoring, elipsometric monitoring, interferometric monitoring or a combination of these techniques. Conventionally, reflectance monitoring measures only the optical thickness (nt) rather than the physical thickness (t). In order to obtain individual values for both the index of refraction (n) and physical thickness, a crystal thickness monitor which varies in oscillation frequency with increasing deposit thickness may be used to measure the physical thickness (t) of the deposited films, permitting calculation of the index of refraction from the reflectance monitor. However, crystal monitors tend to have limited accuracy, and can accommodate only a limited deposit thickness. Removing the deposit or replacing the crystal monitor may require breaking of the deposition chamber vacuum, thereby increasing the potential for impurities in the deposited film.
Elipsometric measurement techniques utilize iterative solution of transcendental functions to obtain values for n and t, which prolongs measurement response time and expense, and presents difficulties in respect to deposition-time error correction. In addition, elipsometric measurement accuracy decreases with increasing film thickness, which conventionally may require the use of witness coupons that must be replaced after a short number of fabrication cycles.
Interferometric monitors are also conventionally utilized to control or monitor layer deposition in the fabrication of optical filters. However, while interferometry potentially has several advantages over reflectance and elipsometric monitoring techniques, conventional interferometric monitoring systems have disadvantages with respect to the efficient, rapid and accurate determination of both refractive index and thickness of the deposited layer(s) of an optical filter during fabrication.
Accordingly, it is an object of the present invention to provide new optical filters, particularly such filters which provide complex optical filtering functions, and methods for designing and fabricating such filters. It is a further object to provide methods and apparatus for rapidly and accurately measuring the thickness and the refractive index of optical filters. It is another object to provide methods and apparatus for fabricating optical filters in which the layer thickness and refractive index are continuously monitored and in which the deposition is controlled in response to the measured thickness and refractive index values. These and other objects will be apparent from the following description and the accompanying drawings.